I'd be more comfortable finding the upper bound estimate, which would be
64 + 4 = 68. 63.75 would normally be rounded off to 64 and 3.6 to 4.
If you want the lower bound estimate, try 63 + 3, or 66. Although not all that far from the actual value of 63.75 + 3.6, which is 67.35, I'm queazy about rounding 63.75 down to 63 when the normal rule would be to round it up.
Results: lower estimate: 66
Actual sum: 67.35
Upper estimate: 68
